Recently, there has existed increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention.
Liquid electrolytes, particularly ion conductive organic liquid electrolytes comprising a salt dissolved in a non-aqueous organic solvent, have been widely used to date as electrolytes for electrochemical devices, such as batteries or electrical dual layer capacitors based on electrochemical reactions.
However, use of such liquid electrolytes may cause degeneration of electrode materials and evaporation of organic solvents, and may be accompanied with a safety-related problem in a battery, including combustion caused by an increase in the ambient temperature and in the temperature of the battery itself. Particularly, a lithium secondary battery has a problem of a so-called swelling phenomenon because of the gas generation in the battery caused by the decomposition of a carbonate-based organic solvent and/or side reactions between an organic solvent and an electrode. Moreover, such side reactions are accelerated under high-temperature storage conditions, thereby increasing the gas generation.
The gas generated continuously as mentioned above causes an increase in the internal pressure of the battery, resulting in deformation of the central portion of a certain surface of the battery, as exemplified by swelling of a prismatic battery in a certain direction. Also, this results in a local difference in the adhesion of the surface of an electrode in the battery, and thus electrochemical reactions cannot occur uniformly on the whole electrode surface and a concentration phenomenon occurs along the thickness direction. Therefore, the battery inevitably undergoes degradation of its quality and safety.
In general, the safety of the battery increases in the order of a liquid electrolyte<a gel polymer electrolyte<a solid polymer electrolyte. However, the quality of the battery decreases in the same order. It is known that batteries using solid polymer electrolytes cannot be commercialized due to such poor battery quality.
However, Sony Corp. (U.S. Pat. No. 6,509,123) and Sanyo Electric Co., Ltd. (Japanese Laid-Open Patent No. 2000-299129) each have recently developed gel polymer electrolytes by using their own unique technological means, and have produced batteries using such gel polymer electrolytes. The batteries have the following characteristics.
Sony's battery uses PVdF-co-HFP (polyvinylidene-co-hexafluoropropylene) as a polymer, and LiPF6 dissolved in EC/PC (ethylene carbonate/propylene carbonate) as an electrolyte. The polymer and the electrolyte are mixed with DMC (dimethyl carbonate) as a solvent, the resultant mixture is coated onto the surface of an electrode, and DMC is allowed to evaporate to provide an electrode onto which a gel polymer is introduced. Then, the electrodes are wound together with a polyolefin-based separator for preventing a short circuit to provide the battery.
Meanwhile, Sanyo's battery is obtained by winding a cathode, an anode and a polyolefin-based separator to provide a cell. Next, PVdF (polyvinylidene fluoride), PMMA (polymethyl methacrylate) PEGDMA (polyethylene glycol dimethyl acrylate), and an initiator are mixed with a suitable organic carbonate mixture, and the mixture is introduced into the preliminarily formed cell. Then, the mixture is crosslinked under an adequate condition to provide a gel polymer electrolyte. In this case, the gel polymer electrolyte is formed in situ inside the battery after assembling the battery.
However, the above batteries have problems in that they are not amenable to mass production, undergo degradation of quality, such as a drop in capacity, and have low mechanical strength.